A large body of evidence implicates dysfunction of mitochondrial homeostasis as a key pathophysiological mechanism in Parkinson?s disease (PD). Maintenance of a pool of healthy functioning mitochondria requires a system for selectively degrading dysfunctional mitochondria (?mitophagy?). Autosomal recessive (AR) PD due to Parkin deficiency links directly to a defect in mitophagy. Mitochondrial dysfunction causes Parkin to translocate to the outer mitochondrial membrane where it interacts with PINK1 (another gene where mutations cause AR PD) to ubiquitinate mitochondrial proteins, thereby inducing fusion of mitochondria with autophagosomes, followed by autophagic degradation. Thus, loss of Parkin leads to the accumulation of dysfunctional mitochondria due to impaired mitophagy. We hypothesize that defective mitophagy also may exacerbate ?-synuclein (?Syn) toxicity. ?Syn induces mitochondrial complex I dysfunction, potentially by directly binding to TOM20 on the mitochondrial membrane and thereby interfering with mitochondrial protein import. Conversely, dysfunctional mitochondria produce increased reactive oxygen species (ROS), consistent with increased markers of oxidative damage in the PD brain. Furthermore, ROS can increase ?Syn accumulation. However, the role of mitophagy in clearing away dysfunctional mitochondria in the setting of ?Syn induced mitochondrial impairment is unknown. Most strategies to modulate mitophagy also alter autophagy in general, or impact other mechanisms, making it difficult to specifically study mitophagy. A target that allows a specific molecular manipulation of mitophagy is USP30. USP30 is a deubiquitinating enzyme (DUB) tethered to the outer mitochondrial membrane, where it directly removes ubiquitin that had been attached by Parkin, thereby counteracting Parkin?s ability to promote mitophagy. USP30 itself is a Parkin substrate, as Parkin normally ubiquitinates and inactivates USP30. Knock-down of USP30 by siRNA rescues mitophagy in Parkin-deficient cells and protects DA neurons in Parkin-deficient Drosophila and protects in an acute toxin model (paraquat). Thus, inhibition of USP30 is an attractive therapeutic strategy for restoring mitophagy to achieve neuroprotection in PD. However, thus far there are no data evaluating the impact of USP30 inhibition on mitophagy in vivo in mammals. Furthermore, the impact of USP30 inhibition has not been tested in a pathophysiologically relevant model of PD. We will address these two issues as outlined in this proposal by testing our main HYPOTHESIS that inhibiting USP30 can serve as an effective strategy to protect dopaminergic neurons against ?Syn toxicity in a progressive degenerative mammalian model of PD.